Frequency modulation



Oct. 13, 1942.

s. L. USSELMAN FREQUENCY MODULATION Filed June 5, 1940 3 Shegts-Sheet lG.- L. USSELMAN I Zmventor (Iitorneg Oct. 13, 1942. e. L. USSELMAN2,293,435

FREQUENCY MODULATION Filed June 5, 1940 3 Sheets-Sheet 3 'I'Dd).

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- c. L USSEL MAN SIGN/9L v SOURCE I (Ittorneg Patented Oct. 13, 1942UNITED STATES PATE NT orrlcs FREQUENCY MODULATION:

George L. Usselman, Port Jefferson, N. Y., assignor to Radio Corporationof America, a corporation of Delaware Application June 5, 1940, SerialNo. 338,837

12 Claims.

signals, telegraphy signals, space wave keying Sig-- The inventiondisclosed here involves nals, etc.

Ilnited broadly the principle disclosed in my States application SerialNo. 338,838 filed June 5, 1940. In the present application I have shownseveral modifications of the system comprising the oscillator, themodulator, and the connections between the oscillator and the modulatortube. The modulation system of this application also involves modifiedphase shifting means in the circuits coupling the modulator to thegenerator.

In describing my invention reference will be made to the attacheddrawings wherein;

Figs. 1 to 6, inclusive, each illustrate a difierent modification of mysystem. Each modification comprises an oscillation generator thefrequency of oscillation of which is stabilized by a piezoelectriccrystal or equivalent means, a tube device, excited by voltagesdeveloped in the oscillater, which is arranged to amplify the saidvoltages and feed them back in phase displaced relation to theoscillator. Means is also provided to modulate the said tube which, forconvenience, has been designated as the modulator tube.

In the modification illustrated in Fig. 1, the oscillation generator isof the electron coupled type, the excitation circuit between theoscillation generator and the modulator tube includes as a phaseshifting means a parallel inductance and capacity, and the modulatortube is of the multi-grid electrode type with the screen grid electrodemodulated. In Fig. 2, the oscillator tube has its main anode and controlgrid coupled in a regenerative circuit and modulating potentials areapplied to the oscillator screen grid as well as to the modulator screengrid. In Fig. 3, the excitation voltage is derived from the circuitcoupled to the main anode of the oscillator tube and the phase shifteris shown by block diagram. In this modification the control grid of themodulator tube is modulated instead of the screen grid. In themodification of Fig. 4, the oscillator and excitation circuits areconnected as shown in Fig. 1, control grid modulation is used. and aparallel tuned circuit is connected with the anode of the modulator tubeand coupled to the oscillation generating circuit crystal.

Fig. is quite similar to Fig. 4 except for the anode circuit of themodulator tube and its coupling to .the crystal in the oscillatorcircuit. The same remarks apply to the modification of Fig. 6 wherein aparticular type of phase shifter in the excitation circuit is alsoshown.

Referring to all of the figures of the drawings except Fig. 2, VI is anelectron discharge device having a grid G3 grounded, grids GI and G2connected in an oscillation producing circuit including thepiezo-electric crystal. The grid G2 is coupled to ground by blockingcondenser E and to a source of-biassing potential by means of a resistorR4. In Fig. 2 the blocking condenser isomitted as is the resistance R4,and grid G2 is 7 connected to the modulation transformer T. In allfigures the control grid GI is connected to one terminal of thepiezo-electric crystal X the other terminal of which is grounded. Thisgrid GI is also connected to round through a potentiometer resistor RIand the cathode K is tapped to a point on RI. The anode III of tube VIis connected.to a tank circuit CI, LI and an output circuit may becoupled to the inductance LI; this output circuit in Figs. 1, 2, 4 and 5comprising a coupling inductance L3, and in Figs. 3'and 6 a couplingcondenser CC.

The modulator tube V2 in Fig. 1 has its cathode 20 grounded andconnected to the negative terminal of a source of potential 25, thepositive terminal of which runs through the secondary winding of atransformer T to the screen grid electrode 34. The control gridelectrode 24 is connected to a parallel tuned phase adjusting circuit LCone terminal of which is connected by a blocking condenser 26 to a pointon resistor Rl to derive excitation voltage therefrom for the grid 24.The anode electrode 40 is connected by a resistor R3 to a source ofpotential not shown. A point on R3 is coupled by a blocking condenser 23to the high radio-frequency potential terminal of the piezo-electriccrystal X, and to the control grid GI. A direct-current circuit for thecontrol grid 24 is completed by a resistance R2.

The arrangement shown in Fig. 2 is somewhat similar to the arrangementshown in Fig. 1. In Fig. 2, however, the oscillator tank circuit LI,which is connected with the anode III of tube VI, has the mid-pointthereon connected to ground by a blocking condenser 3|. This midpointconnection is also connected to theterminal of R3 remote from the anode40 of tube V2 and also to a source of direct-current potential. In bothmodifications the output is derived from a reactance L2 coupled to theinductance LI. In the modification shown in Fig. 2, a coupling capacityC2 is connected between the grid GI and the anode I 0 of tube VI,oscillation generation being produced by virtue of the anode to controlgrid coupling C2. In other words, the oscillator in Fig. 1 is of theelectron coupled type whereas the oscillator of Fig. 2 has its out-- putelectrode coupled to its grid electrode by a capacity. In Fig. 2, theexcitation circuit for the grid 24 of the modulator tube in Fig. 2 isconnected to the high radio-frequency potential end of the crystal X andto the resistance RI which, in this figure, is between the grid GI andground, the cathode K being at ground po- 2 accuse tential. In Fig. 1,the tube V2 only is modulated whereas in Fig. 2 differential modulatingpotentials are supplied by the transformer T to the screen grids 34 andG2 respectively of tubes V2 and VI.

The modification shown in Fig. 3 is somewhat similar to the modificationshown in Fig.1. In Fig. 3, however, the phase shifting circuit P isconnected between the control grid 24 of tube V2 and a point on theinductance LI of the tank circuit LI, CI instead of being coupled to apoint On the resistance RI as in Fig. 1. In Fig. 3 the blockingcondenser 4I is coupled by a variable condenser 44 to the electrode of xand the control grid GI of tube VI. In Fig. 3 the anode 4| derives itsdirect-current potential as in Fig. 1 and this supply of direct-currentpotential is also impressed on the screen electrode 34. The modulationpotentials are applied to the control grid 24 instead of to the screengrid 34 as was the case in the prior figures.

The crystal oscillator circuits of Figs. 4, 5 and 6 are similar to thoseof Figs. 1 and 3 and are of the grounded anode type in which the secondgrid G2 of tube VI acts as the oscillator anode and it is grounded forradio frequency through a by-pass condenser i. The third grid G2 iseither grounded directly or by-passed to ground by a condenser I6 andserves to shield the output circuit from the generating circuits. Bothgrids are supplied with positive potential. In these modifications as inFigs. 1 and 3, the anode of tube VI works into a tuned output circuitCI, LI and is only electronically coupled to the crystal oscillatorcircuit. The crystal X is connected to the control grid of tube VI andto ground. Grid leak resistor RI is also connected to the control gridof tube VI and to ground. The cathode of tube VI is tapped up onresistor RI. It was found that as shown in the circuit of Fig. 6, acondenser C5 connected between the cathode of tube VI and ground aidedor strengthened the oscillations in the oscillation generator.

The control grid of modulator tube V2 in Figs. 4, 5 and 6, as in Fig- 1,is supplied with excitation from resistor RI of the crystal oscillatorthrough an excitation phase shifter P which. in these modifications,serves the function of LC of the prior figures. This phase shifter maybe of any desirable type. The circuit of Fig. 6 shows a series condenserparallel coil combination for the phase shifter unit P. The cathode oftube V2 is grounded, the same as in the prior figures. In Figs. 4, 5 and6, as in Fig. 3, modulation is applied to the control grid of tube V2 byresistance R2 one end of which is connected to the cathode by aradio-frequency by-pass condenser and to the secondary winding oftransformer T. The other end of this transformer winding is connected toa negative bias potential source 25.

The circuit of Fig. 6 includes a switch 35 for connecting the controlgrid 24 to a test bias potential source 45. The primary winding oftransformer T is connected to a signal source A. The screen grid of tubeV2, as in the prior figures, is by-passed to ground for radio frequencyand is also connected to a source of positive potential not shown. Theanode of tube V2, in Figs. 4 and 6, is supplied with positive potentialthrough coil L2, which is connected to a source of positive voltage andcurrent. The lower end of coil L2 is by-passed to ground for radiofrequency through a condenser. In the circuit of Fig. 4

the coil L2 is paralleled by a condenser C2 to form a tuned circuit. Butthe coil L2 in the circuit of Fig. 6 is a half wave electrically, or, inother words, the coil L2 is substantially resonant, and therefore havinga minimum of wattless or circulating current. The high radiofrequencypotential end of coil L2 is coupled to the control grid GI of tube VIand the crystal through a blocking condenser 22 in Figs. 4, 5 and 6. Thecrystal itself forms the blocking condenser between its groundconnection and the control grid of tube VI.

Fig. 5 shows a slight modification of Fig. 4. In Fig. 5 a somewhatdifferent method of supplying the anode 40 of tube V2 with positivecurrent and potential is used. This potential is suppliedlgy means ofresistor R4 instead of through cell The circuits all show differentembodiments of this invention. The circuits of Figs. 1, 3, 4, 5 and 6have crystal oscillator circuits which are substantially the same butthe modulator circuits are somewhat different. In these modificationsthe output circuit of VI is coupled electronically to the generatingcircuits, and shielded therefrom by grounded grid G2. This is not so inFig. 2 where GI is coupled to the anode by condenser C2. The oscillatorsshown in Figs. 1, 3, 4, 5 and 6 are of the grounded anode type ofcrystal oscillator in which the second grid acts as the grounded anode.The real anode works into a tuned output circuit and electronic couplingis used. The crystalx is connected between the control grid GI of tub VIand ground (or G2 potential). The resistor BI is also connected betweenthe control grid and ground, with the cathode tapped up on thisresistor. The tuned circuit C, L of Figs. 1 and 2 in the grid circuit ofthe modulator tube acts as phase shifters. The anode resistor R3 of tubeV2 is tapped with a conductor D and a blocking condenser 22 for couplingthe anode circuit of tube V2 to the control grid GI of tube VI. In Fig.3 the phase shifter network unit P takes the place of the tuned circuitC, L. The crystal oscillator circuit shown in Fig. 2 is of the groundedcathode type using a screen grid tube VI and a tuned anode circuit LI,CI. Anode feedback in the oscillator is'supplied through condenser C2.The screen grid of V2 is modulated in the circuit shown in Fig. 1whereas in Fig. 3 the control grid of tube V2 is modulated. In Fig. 2the screen grids of tubes VI and V2 are modulated in opposition toreduce the amplitude modulation in the output.

The operations of the crystal oscillator and modulator circuits will nowbe described. The modulators in all circuits operate on substantiallythe same principle. In the system of Figs. 1 and 2 the circuit C, L mustbe slightly detuned to act as phase shifters. The action of thesefrequency modulator circuits is as follows, assuming that proper steadypotentials are applied. A small amount of excitation is picked up fromthe oscillator circuit say of like oscillator grid excitation phase bythe tap on RI in Figs. 1, 2, 4 and 6 and the tap on LI in Fig. 3. Inpassing through the phase shifters C, L or P, the phase of theexcitation is retarded say degrees. of course, most phase shifternetworks depend on the terminating resistance or impedance as one of theelements in the mechanismof phase shifting or phase rotation, so thatthe excitation voltage reaching the control grid 24 of tube V2 is saylagging 90 degrees in phase behind the voltage at the oscillator grid.In passing through the The resultant excitation on the grid of tube VIis then leading the normal oscillator excitation.

When the tube V2 is modulated by the signal the amount of modulatingexcitation energy delivered to the grid of tube VI from the anode oftube V2 is also modulated. Consequently, addition of the two componentsof excitation on the grid of tube VI causes the phase angle of thisexcitation to swing between the limits of the two components. This, inturn, causes the frequency to be modulated in accordance'with the signaloscillations.

If in the phase shifting circuits C, L, or P the excitation energy isadvanced in phase. by the same process or reasoning, the modulatingexcitation delivered to tube VI by tube V2 will be retarded in phase andthe resultant excitation on the grid of tube VI will lag the normaloscillator phase. The frequency modulation in this case would be inopposite direction to that described in the first case.

It should be pointed out that the phase .shift in the oscillatorexcitation occurs for each cycle, thus resulting in frequency shift orfrequency modulation. This phase shift continues for each cycle untilthe frequency shift produces an equal and opposite phase shift in thecircuits.

, The modulators shown in Figs. 1 and 3 produce some amplitudemodulation along with the frequency modulated carrier. However, the useof limiters in succeeding stages of the transmitter will eliminate theundesired amplitude modulation from the output.

The frequency modulation circuit shown in Fig. 2 eliminates theamplitude modulation from the frequency modulated carrier output bymodulating the amplitude of the output from tubes VI and V2 in oppositesense. changes generated wave amplitude caused by modulating tub V2 areopposed and compensated by corresponding but opposed modulations on thegrid of VI. That is, if the ground and bias point on the secondarywinding of transformer T is properly chosen, substantially all amplitudemodulation is eliminated from the output circuit in Fig. 2. This wouldalso increase the efliciency of the frequency modulation.

The circuit in Fig. 3 has a condenser 44 which can be used for minoradjustment of phase shift and a switch 48 short-circuiting condenser 44when it is not required. This circuit can be operated with the tap fromphase shifter P on coil LI. The position of this tap on LI depends onthe sense of the phase shift in P. Condenser 44 can be very large, andphase shifter P and condenser 44 can be interchanged. The lead to phaseshifter P can also be connected to a point on RI. A push-pullarrangement may be had by adding another tube V3 excited through anotherphase shifter and coupled at its anode to crystal X by a secondcondenser 44',

The operation of the frequency modulators of Figs. 4 and 6 issubstantially the same. Assum- In this modification ing that propersteady potentials are applied to these circuits, the grounded anodecrystal oscillator operates in the same manner as it has been describedhereinbefore. In both circuits some excitation energy is taken from thecrystal oscillator circuit resistor RI and is delivered to the controlgrid 24 of modulator tube V2 by way of the phase shifter unit P. Thephase of the energy taken from resistor RI is the same as that on thecontrol grid GI of the oscillator tube VI. In passing through the phaseshift network P the phase of the excitation energy is rotated say 90degrees lagging, so that the control grid of tube V2 is, as we haveassumed. excited 90 degrees out of phase, lagging in this case, withrespect to the excitation on the grid of tube VI. In passing throughtube V2 this energy is amplified and reversed 180 degrees in phase.Consequently, the modulating excitation energy fed back to the controlgrid GI of oscillator tube VI from the anode of the modulator tube V2 isapproximately 90 degrees leading in phase com-pared to the voltage ofthe oscillator. If we assume the phase shift in unit 1? to be 90 degreesleading, then the modulating excitation energy delivered from tube V2 totube VI will be about 90 degrees lagging in phase. This phase shift,accomplished in the phase changer network P, may be made any amountdesirable such as,

60, 90, or 120 degrees. However, 90 degrees is assumed to be the bestvalue. Now the excitation on the control grid of the oscillator tube VIis the resultant of the two excitation components, that is one from themodulator circuit and the other from the oscillator circuit. If theamplitude of the modulator (output) component of the excitation energyis modulated in amplitude by the signal, from source A throughtransformer T, then the resultant on the control grid of oscillator tubeVI will also be modulated somewhat in amplitude and what is moreimportant, the resultant excitation will be modulated in phase. Thisresultant excitation phase shift is added to each cycle of theoscillation until an opposite reaction limits the amount of phase shift.Since these are oscillator circuits and the phase shifts are added toeach cycle of oscillation, the resultant modulation is a combination ofamplitude and frequency modulation. When this frequency modulatorcircuit is included in a radio transmitter some amplitude limiter stagesmay be used which will eliminate the undesired amplitude modulation andwhich passes on only the freload on a filter. in this case R2, andcontrol grid capacity of tube V2 are part of the circuit with the filterP affecting the change in excitation energy phase angle.

The undesired amplitude modulation may be eliminated from the output ofthe modulator circuits by introducing some opposite amplitude modulationon the anode of oscillator tube VI through coil LI and through atransformer T2 connected to the signal source A. The amount of oppositeamplitude modulation would have to be just suflicient to cancel andbalance out the 4 amplitude modulation caused by modulator tube What isclaimed is:

1. In a wave generating and wave length modulation system, an electrondischarge device having electrodes including an anode, a cathode and acontrol grid electrode, an oscillation generating circuit coupled toelectrodes of said tube including said grid electrode and said cathode,a piezoelectric crystal in a holder andan impedance in parallel includedin said circuit between said grid electrode and said cathode, an outputcircuit coupled with said anode and cathode, a modulator tube having ananode, a cathode and a control grid, means coupling said modulator tubecathode to said cathode of said device, means coupling said modulatortube anode to said grid electrode, said crystal being in shunt in thecoupling between the anode and cathode of the modulator tube and thegrid and cathode of said device, means coupled with the control grid ofsaid modulator tube for applying thereto voltage of the frequency of theoscillations generated by said oscillation generating circuit, phaseshifting means in said last named coupling, and means for modulating theimpedance of said modulator tube in accordance with control potentialsto thereby modulate the length of the waves generated.

2. In a wave generating and wave length modulation system, an electrondischarge device having a cathode, a grid electrode and an electrodeserving as an anode, an oscillation generating circuit coupled to saidgrid electrode, said electrode serving as an anode and said cathode, apiezo-electric crystal in a holder in said circuit between said gridelectrode and said cathode, an output circuit coupled with saidgenerating circuit, a modulator tube having an anode, a

cathode and a control grid, means coupling said modulator tube anode tosaid grid electrode to which said crystal is coupled, a parallel tunedcircuit coupling the control grid of said modulator tube to a point onsaid oscillation generating circuit at which generated voltage ofsubstantial amplitude appears to apply excitation voltages to saidmodulator tube grid, and means for modulating the impedance 0! saidmodulator tube in accordance with control potentials to thereby modulatethe length of the waves generated.

3. In a wave generating and wave length modulation system, an electrondischarge device having a cathode, a grid electrode and an anode H likeelectrode, an oscillation generating circuit coupled to said gridelectrode, said anode like electrode and said cathode, a piezo-electriccrystal in said circuit coupled to said rid and to another electrode insaid device, an output circuit coupled to said generating circuit, amodulator tube having an anode, a cathode and a control grid, a paralleltuned circuit coupling said modulator tube anode to said grid of saiddevice to which said crystal is coupled, means coupling the control gridof said modulator tube to a point on said oscillation generator circuitat which the generated oscillations are of substantial amplitude toapply excitation voltage of the generated frequency to said modulatortube grid, phase shifting means in one of said last two couplings, andmeans for modulating the impedance of said modulator tube in accordancewith control potentials to thereby modulate the length of the wavesgenerated.

mouse 4. In awave generating and wave length modulation system, anelectron discharge device having an anode like electrode, a cathode anda grid electrode, an oscillation generating circuit coupled to said gridelectrode, said anode like electrode and said cathode, a piano-electriccrystal in said circuit, an output circuit coupled to said generatingcircuit, a modulator tube having an anode, a cathode and a control grid,a reactanoe electrically equivalent to a fraction of the length oi. theoscillations generated for connecting said modulator tube anode to asource of potential, means coupling said modulator tube anode to saidgenerating circuit, a circuit coupling the control grid ofsaid modulatortube to a point on said oscillation'generating circuit at which thegenerated oscillations are of substantial amplitude to apply excitationvoltage of the generated frequency to said modulator tube grid, andmeans for modulating the impedance of said modulator tube in accordancewith control potentials to thereby modulate the length 01 the wavesgenerated.

5. In a wave length modulation system, an oscillation generator of theelectron discharge tube type having electrodes including a cathode and acontrol grid coupled in an oscillation generator circuit including apiezo-electric crystal in a holder having one terminal coupled to thecontrol grid of said tube and having another terminal coupled to thecathode of said tube, said tube having output electrodes coupled in anoutput circuit, an additional electron discharge tube having an anodeand having a control electrode coupled to one of said first two circuitsto derive excitation voltages therefrom, a coupling between the cathodesof said tubes, a couling between the anode of said second named tube andthat terminal of the crystal holder connected to the control grid ofsaid first tube to feed amplified voltage thereto, means for shiftingthe phase of the excitation voltage amplified and fed by said secondtube to said control grid, and means for modulating the impedance of thesecond tube in accordance with signals.

6. In a wave length modulation system, an oscillation generator of theelectron discharge tube type having electrodes including a cathode and acontrol grid coupled in an oscillation generator circuit including apiezo-electric crystal in a holder having two terminals, with a couplingbetween one of said terminals and the grid of said tube and a couplingbetween the other terminal and the cathode of said tube, said tubehaving an anode electrode coupled in an output circuit, an additionalelectron discharge tube having an anode and having a control electrodecoupled to one of said first two circuits toderive excitation voltagestherefrom, a coupling between the cathodes of said tubes, a couplingbetween the anode of said second named tube and the terminal of saidholder coupled to the grid of said first tube to feed amplified voltagethereto, means for shifting the phase of the excitation voltage fed tothe control grid of said second tube, and means for modulating theimpedance of the second tube in accordance with signals.

7. In a wave generating and wave length modulation system, anoscillation generator comprising, an electron discharge device having acathode, an output electrode, and a control electrode an oscillationcircuit coupled with electrodes of said device including said cathodeand control electrode, said oscillation circuit including apiezo-electric crystal in a holder having two terminals with a couplingbetween one terminal and the grid of said device and a coupling betweenthe other terminal of the holder and the cathode of said device, anoutput circuit coupled with said output electrode and cathode, amodulator tube having an anode, a cathode and a control electrode, acoupling between said control electrode of said modulator tube and oneof said first two circuits to derive excitation voltage therefrom, acoupling between the cathodes of said device and tube, a couplingbetween'th anode of said modulator tube and the terminal of said holdercoupled to the grid of said device, phase displacing means in the saidcoupling between the control electrode of said modulator tube and saidone of said first two circuits, and means for modulating the potentialof the control electrode of said modulator tube in accordance withsignals. a

8. In a wave generating and wave length modulation system, anoscillation generator comprising, an electron discharge device having acathode, an output electrode, and a control electrode, an oscillationcircuit coupled with electrodes of said device including said cathodeand said control electrode, said oscillation circuit including apiezo-electric crystal in a holder having two terminals with a couplingbetween one terminal and the grid of said device and a coupling betweenthe other terminal and the cathod of said device, an output circuitcoupled with said output electrode and cathode, a modulator tube havingan anode, a cathode and a plurality of control electrodes, a couplingbetween the cathodes of the device and tube, a coupling between onecontrol electrode of said modulator tube and one of said first twocircuits to derive excitation voltage therefrom, a coupling between theanode of said modulator tube and the terminal of the holder coupled tothe grid org said device, phase displacing means in one ofsaid last twocouplings, and means for modulating the potential on another controlelectrode in said modulator tube in accordance with signals.

, 9. In a wave generating and wave length modulation system, anoscillation generator comprising, an electron discharge device having acathode, an output electrode, and control electrodes, an oscillationcircuit coupled with electrodes of said device including said cathodeand said control electrodes, a piezo-electric crystal in a holder havingtwo terminals with a coupling between one of said terminals and one ofthe control electrodes and a coupling between the other terminal and thecathode of said device, an output circuit coupled with said outputelectrode and cathode, a modulator tube having an anode, a cathode and acontrol electrode, a coupling between the cathodes Of said device andtube, a coupling between said control electrode or said modulator tubeand one of said first two circuits, a tuned circuit coupling the anodeof said modulator tube to the terminal of said crystal holder coupled tosaid one control electrode, phase displacing means in one of said lasttwo couplings, and means for modulating the potential on an electrode insaid tube in accordance with signals.

10, In a wave generating and wave length modulation system, anoscillation generator comprising, an electron discharge device having acathode, an output electrode, and a control electrode, an oscillationcircuit coupled with electrodes or said device including said cathodeand said control electrode, an impedance connected between said controlelectrode and cathode, a piezo-electric crystal in said circuit in aholder coupled between the control grid and cathode of said device, anoutput circuit coupled with said output electrode and cathode, amodulator tube having an anode, a cathode and a control electrode, acoupling between the cathodes of the device and tube, a couplingincluding a phase shifter between said control electrode of saidmodulator tube and a point on said impedance, a

, circuit including an inductance and capacity in parallel tunedsubstantially to the frequency of operation of said generator, aconnection between the anode of said modulator tube and a point on saidinductance, a coupling between said parallel tuned circuit and thejunction point between said crystal holder and the control grid of saiddevice and means for modulating the potential on the control electrodein said tube in accordance with signals.

11. In a wave generating and wave length modulation system, a generatorincluding an electron discharge device having a cathode, an electrodeserving as an anode, and a control electrode, an oscillation generatingcircuit including a piezo-electric crystal regeneratively coupling saidcontrol electrode said electrode serving as an anode and sai cathode forthe production of sustained oscillations, an output circuit coupled tosaid generating circuit, and means for modulating the length of theoscillations generated in accordance with signals in-,- cluding a sourceof si nals, a modulator tube having an anode, a cathode and a controlelectrode, an-excitation circuit coupling said oscillation generatingcircuit to said control electrode of said modulator tube to feed to saidcontrol electrode voltages of the generated frequency,a phase shifter insaid last named coupling for shifting the phase of the voltages fed tosaid control electrode about connections coupling the internal impedancebetween the anode and cathode of said modulator tube effectively inshunt to said crystal, and connections coupling said source of signalsto said modulator tube to modulate the conductivity thereof inaccordance with said signals.

12. In a wave generating and wave length modulation system, a generatorincluding an electron discharge device having a cathode, an electrodeserving as an anode, and a control electrode, an oscillation generatingcircuit including a piezo-electrlc crystal regeneratively coupling saidcontrol electrode said electrode serving as an anode and said cathodefor the production of sustained oscillations, an output circuit coupledto said generating circuit, and mean for modulating the length or theoscillations generated in accordance with signals including a source of.signals; a modulator tube having an anode, a

cathode and, a control electrode, an excitation circuit coupling saidoscillation generating circuit to said control electrode of saidmodulator tube to feed to said control electrode voltages of' thegenerated frequency, connection coupling the internal impedance betweenthe anode and cathode of said modulator tube effectively in shunt tosaid crystal, and connections coupling said source of signals to saidmodulator tube to modulate the conductivity thereof in accordance withsaid signals. t

GEORGE L. USSELMAN.

